This rule will give you warning if you make your Get Credential variables widely accessible than the activity used.
For example here if your Get Credential activity and Username, Password variables are in a single sequence, this is fine. You won’t get any error/warning
If you change the scope of the Username, Password variables to more wider like the Sequence - Parent - You will get this warning/error
So best practice is, get the credential and use it in the same sequence and discard/let it garbage collect as soon as possible.